Viscoelastic Shear Responses of the Cadaver and Hybrid III Lumbar Spine

Due to the sparsity of cadaver lumbar shear stiffness data, tests on functional lumbar spinal units and a complete lumbar section (T12-L5) were done in both the anterior and posterior directions. Similar tests were performed on the Hybrid III lumbar spine for comparison.

Sixteen lumber motion segments were tested quasi-statically for their viscoelastic properties in a multi-directional (5-axis) spine machine. A hydraulic testing machine was used to carry out dynamic tests including cyclic tests at several rates of deformation (0.5 - 50 mm/sec) and relaxation tests (300 sec) to determine the associated viscoelastic properties in constrained and unconstrained modes. The specimens were then loaded to sufficient displacement to cause hard or soft tissue failures.

In the quasi-static tests the shear response was linear and the anterior stiffness (155 ± 90 N/mm) was found to be higher than posterior stiffness (104 ± 38 N/mm). In the relaxation tests the load decreased to approximately 60% of its peak value after 30 seconds. Moderate non-linearity was observed in cyclic loading with shear stiffness up to 750 N/mm, depending on the loading rate. Soft tissue only failures occurred in the unconstrained tests at 1290 N (0.5 mm/sec) and 1770 N (50 mm/sec) for anterior loading. Anterior constrained testing failures involved hard tissue at 2800 N and were not rate dependent. The Hybrid III spine elicited higher initial stiffness than cadaver specimens, but was comparable at shear loads greater than 500 N. It also had considerably greater hysteresis than cadaver specimens.